We have observed a rapid decrease in the rate by which two-dimensional electrons in very high-mobility (μ≊${10}^7$ ${\mathrm{cm}}^2$/V sec) GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As heterostructures are scattered by acoustic phonons as the temperature approaches T=0. This precipitous drop in scattering rate is caused by strong phase-space restrictions for electron-phonon scattering. The characteristic temperature for this transition is not the Debye temperature of the GaAs host material, but the temperature at which phonons of twice the Fermi wave vector cease to be thermally excited.

• Received 18 September 1989

DOI:https://doi.org/10.1103/PhysRevB.41.1278